This likely indicates that you are applying too much rubber. How you apply any brush-on rubber will mean the difference between success and failure when making a brush-on mold.
The key to being successful is to understand that you will be applying the rubber in very thin layers, especially the first layer. If you mix and apply too much rubber on to the model's surface, it will sag or run off of your model. Over applying is a waste of valuable rubber.
Entrapped Air – Brushable rubbers need to de-air themselves, just like pourable ones. Over applying rubber onto the model's surface will result in air entrapment, which will result in air bubbles being evident in the cured mold and any castings made from that mold
It's All In How You Apply the Rubber, Especially The Critical First Layer!
Read the technical bulletin for the rubber you are using before you begin.
To begin, mix a small amount of rubber. If you mix more than you can apply in 15 minutes, what remains in your mixing container will set up and become unusable. You can always mix and apply more during the mold making process if you run short. Pro Tip: Add a small amount of colorant to every other layer to help you find your way and ensure thorough coverage.
For The Critical First Layer – Using a stiff paint brush, apply a thin layer of rubber over the model's surface. Use gravity to your advantage by starting at the top of the model and working your way down to the bottom. Using a "stippling" motion will work the rubber into deep detail.
When you are finished, you should almost be able to see through the rubber to the model's surface. Don't worry, Smooth-On brush-on rubbers blend into themselves nicely and your model will be covered. By having a thin first layer of rubber, it will de-air itself, resulting in a virtually bubble-free mold.
Layers 2, 3 & 4 can be applied using more material, but not too much.
The videos below showcase the proper technique using Rebound™ 25 silicone and Brush-On® 40 urethane rubber as examples: